In conventional Kraft digesters, black liquor is used only in a limited amount, for example, in amounts substantially less than 50% of the total liquid content in the impregnation zone of the digester. The remainder of the externally added liquid usually consists of white liquor. It has now been found that this large addition of white liquor at such an early stage in the cooking process may have an adverse effect on the tear resistance of the fully cooked fibers. Thus, there is a need for a method of digesting pulp which utilizes substantially less amounts of white liquor.
U.S. Pat. No. 3,303,088 (Gessner) discloses a method for continuously cooking cellulose-containing fiber material in a single-vessel system in which:
(1) chips are fed in at a first end of the digester, PA1 (2) white liquor is added at a position at the first end, PA1 (3) the chips are impregnated in a concurrent impregnation zone, PA1 (4) the chips are cooked in a cooking zone downstream of the impregnation zone, PA1 (5) hot black liquor is extracted from at least one screen section, PA1 (6) black liquor is added to the impregnation zone, and PA1 (7) cooked pulp is discharged at the other end of the digester.
This patent also discloses that the extracted liquor from the first screen section, which is arranged downstream of the position of the addition of the black liquor, is returned to the digester by first being conveyed to a container in which white liquor and the extracted impregnation and cooking liquids are mixed. Due to this recirculation, a high content of volatile sulphur and terpene compounds in the impregnation and cooking liquid can build up. Furthermore, the method disclosed in Gessner does not permit sufficiently rapid heating of the cooking liquid to achieve optimal process conditions. It is also evident that the method of Gessner does not include process parameters which are necessary to achieve optimal conditions, such as, the correct liquor-to-wood ratio for obtaining the desire movement of the chip column in the digester.